Recently, projection lithography technology has been developed which employs the EUV (Extreme Ultra Violet) light of shorter wavelengths (11˜14 nm) than those of conventional UV light in order to improve the resolution of optical systems limited by the diffraction limit of light accompanying the increasing fineness of semiconductor integrated circuits. In this wavelength range, transmission/refraction-type optical elements such as conventional lenses cannot be used, and optical systems utilizing reflection are used. Also, reflection-type masks are used as the masks.
As shown in FIG. 6, arranged in front of a mask is a slit substrate 41 having an arc-shaped opening 41a in proximity to a mask M. At the time of exposure, the mask M attached to a mask stage moves as indicated by an arrow, so that patterns formed on the mask surface are illuminated one after another. Note that in FIG. 6, (a) is a side view (scanning direction of the mask M) of the mask M and the slit substrate 41, and (b) is a view of the mask M and the slit substrate 41 from the slit substrate 41 side. The mask M has a pattern region Ma where patterns are formed, and a peripheral region Mb which surrounds the pattern region and has no pattern.
On the other hand, the mask surface is not always flat, and also when the mask is attached to the mask stage, the height-direction position may change, or it may be attached in an inclined state. In such cases, the distance between the mask and the projection optical system changes, by which problems may occur in image-forming performance, such as a blurred image when exposure is performed on a wafer, and errors in magnification or transfer position.
In order to prevent such problems, it is necessary to measure the height-direction position of the mask so that no problem should occur in image-forming performance.
Measurement of the height-direction position of each part of the mask is performed by emitting light from an oblique direction onto a measured surface of the mask and observing the position where light reflected by the mask surface enters a receiving surface, because the observed light incident position changes if the height of the measured surface of the mask changes, the height of the measured surface of the mask can be measured.
However, when attempting to measure the height-direction position of the exposure surface of a mask during an exposure time by the optical method in an EUV exposure device having the slit substrate, there is the problem that the slit substrate becomes an obstacle. In FIGS. 6(a) and 6(b) when measuring the height-direction position of the mask M surface, as shown in FIG. 6(a), light 43 is emitted from a light source 42, and the position of light reflected by the mask M surface is observed by a photo-detector 44. Although only one beam of light 43 is shown in FIG. 6(a), in actuality multiple slit images 45 are formed as shown in FIG. 6(b), which multiple height-direction positions are simultaneously measured.
However, when the slit substrate 41 exists, the light is obstructed by the slit substrate 41 and does not reach the mask M surface. Namely, as shown in (b), among the slit images 45 (shown in the figure are 9×2 slit images as an example), although some of them pass through the opening 41a of the slit substrate 41 and reach the mask M surface, others are obstructed by the slit substrate 41 and do not reach the mask M surface, in which state the height-direction position of the mask M surface cannot be measured.
Although the above explanation was made regarding a method of measuring the height of the mask M surface using light, also in the case of using height measurement devices of a contact type or non-contact type where other principles such as electrostatic capacity, eddy current, and ultrasound are applied, problems may occur in which measurement is made impossible by the existence of a slit substrate 41 or the measurement data do not satisfy a specified accuracy. Also, even in the case of using a transmission mask is used instead of a reflection mask, various problems occur when an exposure area defining member such as a slit substrate which defines an exposure area is used between the mask and the projection optical system.